linux-kernel - Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

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Date:	Mon, 20 Aug 2012 11:59:11 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Rafael Aquini <aquini@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

On Wed, 15 Aug 2012 17:40:19 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Wed, Aug 15, 2012 at 09:34:58AM -0300, Rafael Aquini wrote:
> > On Tue, Aug 14, 2012 at 10:31:09PM +0300, Michael S. Tsirkin wrote:
> > > > > now CPU1 executes the next instruction:
> > > > > 
> > > > > }
> > > > > 
> > > > > which would normally return to function's caller,
> > > > > but it has been overwritten by CPU2 so we get corruption.
> > > > > 
> > > > > No?
> > > > 
> > > > At the point CPU2 is unloading the module, it will be kept looping at the
> > > > snippet Rusty pointed out because the isolation / migration steps do not mess
> > > > with 'vb->num_pages'. The driver will only unload after leaking the total amount
> > > > of balloon's inflated pages, which means (for this hypothetical case) CPU2 will
> > > > wait until CPU1 finishes the putaback procedure.
> > > > 
> > > 
> > > Yes but only until unlock finishes. The last return from function
> > > is not guarded and can be overwritten.
> > 
> > CPU1 will be returning to putback_balloon_page() which code is located at core
> > mm/compaction.c, outside the driver.
> 
> Sorry, I don't seem to be able to articulate this clearly.
> But this is a correctness issue so I am compelled to try again.

But if there are 0 balloon pages, how is it migrating a page?

> In the end the rule is simple: you can not
> prevent module unloading from within module
> itself. It always must be the caller of your
> module that uses some lock to do this.

Not quite.  If you clean up everything in your cleanup function, it also
works, which is what this does, right?

Cheers,
Rusty.

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